Conventionally, cameras, video cameras, and the like employ solid-state image pickup devices. As such a solid-state image pickup device, a CCD (charge-coupled device) image sensor or a CMOS (complementary metal-oxide semiconductor) image sensor is used. Such an image sensor is provided with a fine converging lens (hereinafter referred to as “microlens”) designed to improve light-gathering power.
Such a microlens is formed with use of a positive-type photosensitive resin composition. Specifically, first, a photosensitive resin layer is formed from a photosensitive resin composition. Then, the photosensitive resin layer is partially exposed. Next, by means of development, a pattern is formed by removing the exposed portion. After that, the photosensitive resin layer is fluidized by treating the pattern with heat, and then is shaped into a lens (hereinafter also referred to as “thermal flow”), with the result that a microlens is obtained.
In a process for manufacturing an image sensor, other components such as a color filter are formed as needed after the formation of a microlens. Such components as a color filter are formed by being treated with heat under temperature conditions of approximately 230° C. to 250° C. However, a micros lens formed with use of a lens-forming photosensitive resin composition composed mainly of a hydroxystyrene resin has such insufficient heat resistance as, for example, to become lower in transmittance when treated with heat.
In order to solve such a problem, Japanese Unexamined Patent Application Publication No. 251464/2006 (Tokukai 2006-251464; published on Sep. 21, 2006) discloses a lens-forming photosensitive resin composition excellent in heat resistance and transparency. The lens-forming photosensitive resin composition contains a thermal crosslinker. This makes it possible to form a microlens with improved heat resistance in comparison with the lens-forming photosensitive resin composition composed mainly of a hydroxystyrene resin.